This invention relates to a printing device having a plurality of characters arranged in at least one line circumferentially extending on the peripheral surface of a printing drum, and more particularly the type additionally provided with an apparatus for confirming the correct impression of printing characters which can recognize a coincidence between printing input data and any character actually impressed on a recording medium.
A drum type printing device wherein printing characters are circumferentially arranged in a plurality of lines at a prescribed peripheral interval impresses desired characters successively on a recording medium by selecting the angle through which the printing drum is rotated.
The printing drum is coupled with a pulse motor driven to an extent corresponding to the selected rotation angle of the printing drum. Control of the rotation steps of the pulse motor is effected by calculating a number of the rotation steps of the printing drum required to bring printing characters corresponding to input data supplied by manual operation or, previously preset in a memory or transferred signals of printing characters to a prescribed printing position and stepwise driving the pulse motor in accordance with the calculated rotation steps. Where, with the prior art printing device, a signal denoting, for example, a printing character "B" corresponding to a rotation angle II of the printing drum is supplied after impression of, for example, a printing character "A" corresponding to a rotation angle I of the printing drum, then comparison is made between the rotation angle II of the printing drum relative to the character B being impressed next time and the rotation angle I of the printing drum corresponding to the previously impressed character A. The pulse motor is driven by a number of step signals required for the printing drum to be rotated to an extent corresponding to a difference between the above-mentioned rotation angles II and I, thereby bringing the character "B" type to a printing position.
Namely, a step signal for causing the pulse motor to be driven in a sufficient number of steps to bring a desired character type provided on the printing drum to a printing position is issued, each time said pulse motor is supplied with a signal denoting a desired printing character. Said step signal controls the rotation of the pulse motor, thereby determining the rotation angle of the printing drum. With the above-mentioned conventional drum type printing device, the printing drum is rotated in accordance with the above-mentioned step signal, effectively carrying out high speed impression and rendering said printing device very useful as an output device for an electronic computer.
With the prior art printing device, however, the printing drum 1 is rotated at each time of impression to an extent corresponding to a difference between a rotation angle relative to the preceding character and that corresponding to the succeeding character without being brought back to the prescribed point of 0.degree. reference angle.
Where, therefore, noncoincidence occurs between a step signal and an actual amount of rotation of the pulse motor for rotating the printing drum due to, for example, mechanical slips or absence of signals, then characters subsequently impressed on a recording medium do not match input character signals. Particularly where printing input data consists of for example, a series of numerals, notations, or combinations thereof, a mere look at an impression on a recording medium fails to find printing errors if any. In such case, the impression becomes little reliable.
With the drum type printing device, printing characters on the drum are selected not only by the rotation of the drum but also by its axial movement. The axial movement of the drum is carried out by a step signal instructing comparison between a number of steps by which the previously impressed character line is spaced from the rear side of the printing drum and a number of steps by which a new character line being impressed is spaced from the rear side of the printing drum and also the advance of a printing hammer. When new printing input data is supplied, the above mentioned comparison is made by, for example, a matrix circuit, issuing a signal denoting the result of comparison, or a balance arrived at by subtraction between the above-mentioned two numbers of steps, that is, an extent to which the printing drum is required to move axially for impression of a new character line. This axial movement is effected by the pulse motor or ratchet mechanism. Therefore, a selected character is correctly impressed, as long as the printing drum is driven to an exactly desired extent. Where, however, non-coincidence occurs between a step signal issued and an actual amount of the printing drum due to, for example, mechanical slips, absence of signals and intrusion of noises, then all the subsequently selected character lines do not exactly correspond to input character data, failing to attain correct impression. Particularly where the above-mentioned prior art printing device is used as output means with, for example, an electronic computer, it is difficult quickly to recognize errors of impression. Where the impression consists of, for example, a series of numerals, notations or combinations thereof, a mere look at the impression fails to find errors.